scala - Parboiled2 PopRule example -

after reading documentation on https://github.com/sirthias/parboiled2, discovered pop out of stack rule:

type poprule[-l <: hlist] = rule[l, hnil] 

but not find working example of type of rule when l not string.

for example, supose have following rule:

case class a() case class b() def foo = rule { push(a) } def pop_rule:poprule[a, hnil] = rule { pop(a)} 

to justify there general definition of parboiled2 rule:

class rule[-i <: hlist, +o <: hlist] 

where represents rule pops value from stack , puts value o stack.

so far, cannot think of example implementation following rule type:

def rule_of_interest:rule[a, b] = rule { pops(a) ~> push(b)}